This invention relates generally to actuators for railway hydraulic braking systems, to an actuator for applying the service and parking brakes in such systems and, more particularly, to a slack adjuster and a brake wear compensator for such an actuator.
Railway braking systems have traditionally been operated by pneumatic actuators. These air brake systems are still in common use on long distance railways and on many subway systems. Such air brake systems are effective, but lack the ability to modulate braking effort sufficiently to provide consistently smooth stops.
Recently, braking requirements for rapid transit railway cars have resulted in the development of hydraulic friction service braking systems which are controllable to provide the desired smooth stops. Such a hydraulic braking system is disclosed in U.S. Pat. No. 29,096 to Jones. This patent relates a hydraulic friction braking system and a control system of a type currently in use on Washington Metropolitan Area Transit Authority (WMATA) transit cars. This system employs a brake actuator which hydraulically applies and releases the service brakes.
Parking brakes are required to apply and maintain the brakes applied for extended periods when the railway car is out of service and parked. Parking brake actuators have traditionally been mechanically applied and held. A hydraulic parking brake actuator has been developed, for use in the WMATA braking system, as shown in U.S. Pat. No. 4,083,436 to Straut. In this actuator, hydraulic pressure is utilized to extend a piston and apply the parking brake, while a mechanical lock holds the parking brake engaged. This mechanical lock is hydraulically released to enable a spring to retract the piston and release the brake.
In the Straut arrangement, a railway hydraulic actuator assembly includes separate actuators for applying both the service and parking brakes of a railway car. The service brake actuator applies the service brakes by spring force applied through service brake pistons; application of hydraulic pressure fluid, supplied from a pump-fed accumulator, releases the brakes.
A separate actuator applies the parking brake via hydraulic pressure created by a hand pump acting on a separate parking brake piston. The brake is held applied by a mechanical lock comprising a complex friction clutch which locks the piston extended when hydraulic pressure is removed; hydraulic pressure is again utilized to release the friction clutch and to enable the spring force to retract the piston and release the brake.
Although hydraulic railway service and parking brake actuators according to the above patents have been used successfully for many years, they are unduly bulky, complex and expensive to purchase and maintain. Accordingly, it would be desirable to reduce the complexity of railway hydraulic brake actuators.
Further, it would be desirable to provide a parking brake actuator having a mechanical lock of reduced complexity.
In addition, it would be desirable to combine the parking brake and service brake actuators into a single actuator.
Hydraulic railway braking systems have utilized a rotary brake wear compensator in the form of a buttress nut and large lead screw arrangement to automatically compensate for brake wear, such as that illustrated in the Straut patent and in U.S. Pat. No. 3,995,722 to Jones et al. This is a cumbersome and expensive arrangement.
Accordingly, it would be desirable to provide a hydraulic railway service brake actuator having an automatic brake wear compensator of less complexity than the automatic rotary device in current use.
Various forms of slack adjusters have also been used. The Jones patent illustrates the use of a one-way clutch, in the form of a sprag clutch, to provide a predetermined amount of brake wear, while allowing for brake wear adjustment. Unfortunately, these sprag clutches have a failure mode in which the sprags can fail over-center, locking up the brakes. This failure is considered catastrophic in the sense that it prevents the failed brakes from being cut out, thus immobilizing that car and its train.
Accordingly, it would be desirable to provide a slack adjuster that is simpler and does not have a catastrophic failure mode.